Forecast Updated Wed 8:03 PM — This morning's update captured the trend towards lower snow totals.

Tonight's range models will begin to become available over the next hour (NBM, HRRR, SREF, RAP, NAM, NAM-NEST and HIRESW).   

The late afternoon trend continues with the main axis of precipitation falling from Philadelphia and south; these same areas will have the earliest transition to sleet/freezing rain.
  
This afternoon's Canadian Regional GEM and German ICON model show QPF values of 0.3-0.4 inches water (3-4 inches of snow) prior to the transition to sleet and freezing rain. 

I hope to have an update by 9:30 PM.

Forecast Updated Wed 5:10 PM — Quick update. Precipitation values have reduced from yesterday's models.  3-4 inches is the more likely accumulation from the city north.  More details this evening.

Forecast Updated Wed 8:27 AM — The models are still predicting in the general 4-6 inch range of snow accumulation especially north of PHL, followed by sleet and freezing rain.   Some of the models that are run at 1 AM EST in the morning (the 06z Models) are showing a more southern track with less snow.   

Another issue with snow accumulation not to be ignored is the average temperature of the atmosphere up to 18,000 feet (as depicted by a derived parameter called the "1000-500mb thickness" ) is marginally too warm for most snowstorms in our region.   While it will be cold in the lower layers, the setup really supports sleet and freezing rain.    My real day job will take precedence over these forecasts today, but I'll be back this evening for an update. 

Forecast Updated Tue 10:24 PM — Tonight's short range models have become available still show a heavy punch of snow in the morning, starting before 7AM Thursday followed by significant sleet and freezing rain. About 5-7 inches of snow before the changeover about 1PM.  Northwest areas of Montco and Bucks may stay all snow with higher accumulations.  

There have been some shifts in the model forecasts. The snow will start a little earlier than previously forecast starting as early 4-6 AM Thursday from southwest to northeast.

The NAM and NAM-NEST are showing a heavier snow accumulation in the morning prior to a change to snow/sleet and eventually sleet.  

The Short Range Ensemble Forecast (SREF), which combines statistical variations of the short range models including the NAM, has quite a spread in the possible snow accumulations and snowfall timing, as depicted in this simple one location meteogram for Blue Bell—

Capturing the spread and uncertainty is the latest Model Blend  (NBM) snowfall totals—

My preferred model for snowfall/sleet has always been the NAM model or a combined NAM-GFS blend.  Here’s the latest NAM (which tends to run high this much in advance)—

The NAM is showing an early burst of heavy snow during the morning.

Additionally, the NAM has an extended period of sleet and freezing rain on top of the snow after 12-1 PM.

This afternoon's GFS model has just become available as I write this.  The possibility of at least a 5-8" generalized snowfall is forecast with the snow -> sleet changeover delayed into the late afternoon.  

As usual, we have uncertainty with a coming snowstorm.  But the trend is for a more significant snow accumulation.    Stay tuned for updates.

Forecast Updated Mon 5:24 PM — It still looks like all rain for tonight.  (see meteogram below)

Thursday's storm look like snow, quickly changing to sleet and then all rain, according to the NAM.  Starts Thursday morning about 7-9AM.

Forecast Updated Mon 7:19 PM —The GFS has more snow than the NAM prior to the changeover.

From Sunday…

The forecast has been difficult regarding specifics beyond a day or two in advance this winter.  That said, I think I can try to pin down the wintry and rainy forecast for Monday into Tuesday.

First, tonight there may be another period of very light freezing rain about 4AM which should taper to widely scattered precip  after daybreak.   During the day, Monday, there may be some sprinkles, but most of the day will be dry.

Low pressure will approach later Monday and affect our area Monday night into Tuesday morning. Instead of a coastal storm as forecast last week, Monday into Tuesday’s storm will move up the Appalachian mountains and no coastal development is expected.

According to the Model Blend (NBM), surface temperatures will be above freezing as early as the early afternoon Monday and remain above freezing. Any sleet or mixed rain during the late afternoon shouldn’t accumulate.

I’m always looking for ways to convey the forecast more succinctly.  What might best capture this event is a simulated radar time-line (a “meteogram”)  showing light possible mixed precipitation before 8 PM, then increasing heavier precip until 9-10 AM Tuesday.

Of greater interest is a large storm expected beginning Thursday morning.  Another complex precip event which may begin as snow, but the GFS is pointing to secondary low pressure development off the coast which may bring a change back to all snow.   Much uncertainty with this storm.

Those of you getting tired of this weather can look towards next week, where the long range models are showing milder and less stormy conditions taking hold.

Thu 04:15 PM Forecast Review — Here are the official NWS snow totals for the immediate PHL area for Wednesday night’s snowfall.

The GFS and Canadian GEM were too high, the NAM close but a bit low.

Forecast Updated Wed 10:50 PM —Tonight’s GFS is unchanged from this afternoon’s model run and remains about 1 inch greater than the NAM posted just below.   In past years, an average between the GFS and NAM models has worked. We’ll know by mid to late morning tomorrow.

Forecast Updated Wed 9:45 PM —Tonight’s HRRR, RAP, NAM and NAM-NEST models just became available and they have brought down the snow totals a bit from the afternoon models.  Here’s the latest NAM—

Also available at this hour is the new WRF-ARW, whose snow totals are similar to the afternoon ranges below.   So I think we’re splitting hairs here, as we all know that the models can’t really be this precise.  I think we’re back to the 2-4″ range for the immediate Philadelphia area, less further north. 

The new GFS and Canadian models won’t be available until after 10:30 PM.  If I’m still awake, I’ll update! 🙂

Forecast Updated Wed 5:26 PM — The models are on-board with snow for tonight (Wednesday), developing between 8 and 10 PM and ending about 10 AM Thursday.   This afternoon’s GFS has just come out (I’m using the soon-to-be-released GFS v.16 here)—

To present another viewpoint, here’s the latest Canadian Regional “GEM” model—

It still looks like the Friday storm will miss us, passing by to our south.  Another, more-developed storm is now scheduled for Saturday afternoon instead of Sunday.  This also looks like the bulk of its snow will pass to our south, but some accumulation is expected here. Stay tuned for updates.

Forecast Posted Wed 9:10 AM —

The latest models have converged on a similar snow accumulation forecast for Wednesday night into Thursday morning.  (At least that’s the case right now. )

The impulse moving in from the west will pass just to the south of us.  We’ll be on the cold side of the ‘storm’.   As mentioned last night, the somewhat greater accumulations will be just south of Philadelphia.

Things may change, as they always seem to do.  That said, there’s pretty good agreement  with these models.

As for the second storm that had been expected to affect us later Thursday into Friday, it appears it will be moving to our south, essentially missing us.

There’s additional uncertainty about the weekend storm; its track seems to be further south as well..  Stay tuned.